The affinity of oxygen for engineering alloys used in the industrial water industry is the cause of many complex corrosion phenomena, which not only depends on the amount of oxygen in a system, but also on factors such as the water chemistry, system design, and metallurgy. For example, the presence of other species in the water could turn oxygen into an aggressive corrosive force, or could render the metallurgy passivated. Additional important factors include temperature, pressure, fluid velocities, and other operational practices. While oxygen might be a primary or essential component in the corrosion process, it is usually not the only one.
Conventional means for reducing oxygen corrosion in water systems is to remove most of the molecular dissolved oxygen (“DO”) by mechanical and chemical means. The vast majority of DO can be reduced into the ppb regime by the use of mechanical deaeration, where the water is typically heated to above its boiling temperature in a vented vessel. The solubility of the DO in water decreases as the temperature increases. Flow dynamics and operational issues particular to deaerators (“DA”) leave some DO in the water—typically sub parts per million. The chemicals used to reduce DO values further to reproducibly low and constant values are called oxygen scavengers. Many of these scavengers also function as passivating corrosion inhibitors.
Deaerators do not always work perfectly. If they did, a pure scavenger might never be needed, although a chemistry regime that enhances metal passivation would be a positive addition. Thus, in some cases, the oxygen scavenger is added as an insurance policy against the possibility that the DA might malfunction. The scavenger can also be added to combat air in-leakage, which might occur at boiler feedwater (“FW”) pumps, for example.
In the case of boilers, the amount of oxygen scavenger fed to the boiler FW has been traditionally based on the amount of DO in the FW plus some excess amount of scavenger. The amount of excess scavenger fed is typically based on the desired residual scavenger concentration in the boiler FW or boiler water itself, which is a function of the excess concentration of scavenger and boiler cycles. Conventional wisdom teaches that oxygen scavenger feed varies linearly with system flow rates and scavenger pumps can be “slaved” to system flow rates.
Several problems exist with this feed control scheme. First, is the absence of active control of scavenger feed rate. Instead, implied feed is based on system flow rates. High DO conditions could exist for long periods of time before a decrease in scavenger residual occurs and corrective action is taken. A second issue is that the presence of residual scavenger in the boiler water simply does not mean that the system is being treated satisfactorily. Depending on the conditions (e.g., low temperature or short residence time) it is possible to have both high oxygen concentrations and sufficient scavenger in the FW at the same time. While oxygen attack might be most aggressive in the FW system including the boiler FW heaters and economizer regimes, water containing DO can reach the boiler. When this oxygen rich FW reaches the boiler, oxygen is flashed off with the steam leaving the unreacted scavenger in the boiler water. In the extreme case, resulting in unacceptably high DO levels in the pre-boiler and condensate systems while having the expected residual concentrations of oxygen scavenger in the boiler itself.
Typically, for plants using sulfite-based oxygen scavengers, a sulfite residual is maintained in the boiler itself that might be checked once per shift. Boiler FW sulfite residuals are generally not measured and are unknown. Plants using other DO scavengers/passivators, such as hydrazine, carbohydrazide, erythorbic acid, diethylhydroxylamine, methylethylketoxime, hydroquinone, and the like, a fixed scavenger residual is generally maintained in the boiler FW. Residuals might be checked once a shift or online. Control is relatively straightforward in these systems and can be effective in “stable” systems. There is generally no response to “short-lived” changes in chemical demand and also no correlation between product feed and real-time system corrosion potentials. Although rare in most industrial boiler FW systems, DO concentrations might be measured. In the vast majority of cases, oxygen scavengers are fed at a constant feed rate and residuals are measured periodically. In this type of scheme, scavenger residuals are based off historical data (using grab samples), which typically misses the dynamic FW reduction-oxidation stresses.
The next more sophisticated approach is to feed scavenger based off a linear equation relating scavenger pump speed to boiler FW flow or steam load. This approach appeals to one of the stresses experienced by a DA—namely boiler FW flow. The problem is that it does not address input variation stresses (makeup versus condensate return flow into the DA and DO variations of the incoming DA feedwater). One does not know if scavenger is fed into the system unless residuals are measured.
Tracer chemicals can also be formulated with other “active” boiler water chemicals in different ways. For example, the feed of the active chemical may not be measured; rather, the feed of a traced species is measured. When the specific formulation is known, it is possible to calculate the active chemistry based upon the measured tracer signal. This method usually involves basing scavenger feed off an inert tracer signal (e.g., adding an inert tracer chemical to the active scavenger). Operators would need to check scavenger consumption from time to time and should also measure DO from time to time. The feed of scavenger could also be slaved to a boiler water internal treatment tracer signal. Tracer concentrations would vary linearly with flowrate. Still, a user would not know if scavenger were getting into the FW unless a residual test was run. Scavenger need might not be linearly proportional to FW flow. From an oxidation-reduction potential standpoint, it is not. Instead, this need is related to other factors, such as how much the DA is being stressed for the actual FW flow at any given time.
Scavenger feed may also be tracked with traced actives. For example, gallic acid can be traced as an oxygen scavenger itself (See U.S. Pat. Nos. 6,436,711 B1 and 6,566,139 B2). In this case, one knows what is going in (pump speed) and how much active is present in the system. It does not address, however, the amount needed based on DO variations, especially where the scavenging reaction is not complete at the point of measurement. Tracing both actives and oxygen scavenger products has also been practiced, for example, with gallic acid. This scheme still does not directly address the DO variations in real-time terms and the equilibrium that exists between the unreacted scavenger, dissolved oxygen, and reacted scavenger.
There thus exists an ongoing need for better and more efficient methods of managing and controlling active chemical species to maintain low corrosion conditions in industrial water systems. Current wisdom is to feed such species at concentrations linearly with respect to FW flow. This linear scheme is inadequate when trying to control ORP, especially without ORP measurement devices. An ideal method would include determining an oxygen scavenger feed into the system using a nonlinear control scheme based on system variables such as those described below.